The present invention relates to a microwave excitation system and more specifically to a system which couples a magnetron to a waveguide of relatively compact dimensions with no intermediate or transition waveguide structure. The present invention is an improvement of the excitation system disclosed and claimed in commonly-assigned U.S. Pat. No. 4,316,069 to the inventor of the present invention.
The above-referenced '069 patent discloses a microwave excitation system which provides an improved time-averaged energy distribution pattern for a microwave oven. That excitation system employes a circular waveguide section attached in a centrally located position to the bottom wall of the microwave oven cooking cavity. A rotatable antenna having a vertical segment and a horizontal arm is rotated to excite TM.sub.01 the mode by the vertical segment and the TE.sub.11 mode by the horizontal arm. These two modes interact to excite a rotating asymmetric electric field within the waveguide. This waveguide field is propagated into the oven cavity to produce a rotating field pattern of desirable time-averaged uniformity in the cavity during the cooking cycle. The excitation system further includes a rectangular waveguide section which couples microwave energy from the magnetron to the rotating antenna.
It would be advantageous to have the magnetron couple directly into the circular waveguide section, thereby eliminating the manufacturing and materials costs and the energy losses associated with the rectangular waveguide section, while retaining the radiating pattern provided by the system of the '069 patent. One difficulty with coupling the magnetron antenna probe directly into the circular waveguide results from the fact that because of the relative compact waveguide dimensions dictated, at least in part, by packaging and material cost constraints, and the required radiating probe length for satisfactory probe coupling, the probe may well have to extend beyond the waveguide into the cooking cavity itself.
Examples of excitation systems for microwave ovens which have eliminated the external rectangular waveguide are disclosed in U.S. Pat. No. 4,350,859 to Dudley et al and U.S. Pat. No. 4,105,886 to Baron et al. However, neither of the above-described excitation systems provides a radiation pattern in the oven similar to that of the '069 patent. In Dudley et al, the magnetron probe is inserted into a circular well extending from the oven cavity. A rotating feed structure comprising three waveguide sections is rotatably supported from the magneton probe. Each of the three waveguide chambers on the rotating structure has associated with it a radiating slot. Thus, the magnetron probe couples into the waveguide chambers of rotating waveguide structure with each chamber coupling energy to the cooking cavity via its associated radiating slot.
In Baron et al, a square housing which opens into the bottom of the oven cavity houses the magnetron probe in one corner thereof with a rotating mode stirrer centered in the housing. The energy propagating in the housing is deflected into random patterns by the mode stirrer which enters the cooking cavity. The corner location for the probe is probably dictated by magnetron loading requirements. One consequence of this location of the probe which is off-center relative to the oven cavity is that the resultant radiation patterns established in the cavity may be asymmetric with respect to the cavity, resulting in more energy being delivered to one part of the cavity than another, depending upon the effectiveness of the mode stirring arrangement in randomizing the patterns.
In view of the potential advantages associated with elimination of the external rectangular waveguide, it would be desirable to provide a microwave excitation system which eliminates the costs and energy losses associated with the external rectangular waveguide while providing a radiation pattern comparable to that provided by the '069 patent.
It is therefore an object of the present invention to provide a microwave excitation system particulary adaptable to microwave ovens which is of relatively simple, low cost construction, and which permits coupling the magnetron to a waveguide of relatively compact dimensions without using an external rectangular waveguide intermediate the magnetron and the waveguide for coupling energy from the magnetron to the guide.